Information concerning respiration and heart function of an animal such as a human (a “subject”) is useful for diagnosis and monitoring of physiologic (e.g., medical) conditions, as well as confirming presence of a live subject. Various instruments have been devised to sense physiologic activity, but such instruments typically either impede movement of the subject, are inconvenient to use, or are non-portable in character.
An electrocardiograph is a device that is commonly used to provide information, often in the form of an electrocardiogram, concerning heart function. Electrocardiographs provide outputs that are indicative of electric fields created by the heart as it beats. Operation of an electrocardiograph typically requires attachment of nine leads, which are combined to obtain twelve sets of measurements. A large body of clinical experience has revealed correlations between specific shapes in the waveforms output by an electrocardiograph and many different types of heart conditions.
An impedance cardiograph is another device that is used to provide information, often in the form of an impedance cardiogram, concerning heart function. Impedance cardiographs measure changes in impedance within tissue to estimate changes in volume of a subject's body and organs. When alternating currents are transmitted through a subject's chest, the impedance of the tissue in the patient's chest is altered with changes in blood volume and velocity in the aorta according to each beat of the subject's heart.
A phonocardiograph is a device commonly used to provide detailed information on heart sounds, usually in the form of a phonocardiogram. The phonocardiogram waveform is measured by placing a sensitive microphone, or accelerometer, in contact with the chest at one of several well-defined auscultation locations.
Electrocardiographs and impedance cardiographs typically involve attaching electrical leads to the subject being measured, and impedance cardiographs typically involve passing a current through the subject's body. Phonocardiographs require attaching a specially-designed microphone or accelerometer to the subject's torso.
As an alternative to the foregoing cardiac sensing instruments that require electrical leads, or vibratory or accelerometric sensors, U.S. Pat. Nos. 6,122,537; 5,760,687; 4,958,638; 6,753,780; 6,208,286; 6,031,482; and 5,488,501 (which are hereby incorporated by reference herein), demonstrate modulation of the phase and/or frequency of a reflected radio frequency signal (i.e., radar or Doppler radar techniques) to provide a measurement of pulse rate and/or respiration rate. A target with time-varying position but no net velocity will reflect the signal, modulating its phase in proportion to the time-varying position of the target. A stationary person's chest has periodic movement with no net velocity, and a continuous wave radar apparatus trained on a person's chest will receive a signal similar to the transmitted signal with its phase modulated by the time-varying chest position. A signal proportion to chest position can be obtained by demodulating the phase-modulated signal.
More recently, systems and methods for remotely sensing cardiac-related data of subjects were disclosed in U.S. Pat. Nos. 7,811,234 and 7,272,431. U.S. Pat. No. 7,811,234 discloses a non-imaging method of remotely sensing cardiac-related data of a subject, the method including: transmitting a microwave signal to illuminate tissue of the subject; receiving a reflected microwave signal, the reflected microwave signal being a reflection of the microwave signal from illuminated tissue of the subject; processing the reflected microwave signal and analyzing an amplitude of the reflected microwave signal to determine changes in a reflection coefficient at an air-tissue interface of the subject's body resulting from changes in permittivity of the illuminated tissue of the subject, the changes in permittivity containing a static component and a time-varying component; and processing the time-varying component to provide cardiographic related data of the subject.
Applicants have found that consistent reproducibility of remote sensing methods according to U.S. Pat. No. 7,811,234 may be highly sensitive to factors such as: (i) relative position (e.g., angular position) between a radio frequency signal transmitter and a corresponding receiver; (ii) movement (whether voluntary or involuntary) of a subject; and/or (iii) presence of interfering signals. Transitions in heart rate, such as when a subject has initiated or terminated exercise, can increase the difficulty of remote cardiac sensing methods. Moreover, for reasons not yet fully appreciated, specific signal processing schemes useable with remote cardiac sensing may work relatively well for certain groups of individuals but not work consistently well with other individuals outside the group.
It would be desirable to facilitate consistently reproducible results for remote sensing (i.e., without contacting an animal subject) of physiologic activity despite presence of one or more complicating factors as outlined above. It would also be desirable to promote efficient utilization of medical diagnostic and treatment resources.